European Patent Specification No. 0,106,379 discloses a process in which an olefinically unsaturated compound is carbonylated with carbon monoxide in the presence of water, and alcohol and/or a carboxylic acid, a palladium catalyst, at least 5 mol or a phosphine PR.sup.1 R.sup.2 R.sup.3 in which R.sup.1, R.sup.2 and R.sup.3 each represent an optionally, substituted aryl group, per gram-atom of palladium, and, as promoter, an acid with a pKa&lt;2 (at 18.degree. C. in aqueous solution), except hydrohalogenic and carboxylic acids.
Research carried out by the Applicant into these processes has shown that the activity of the catalyst composition can be considerably enhanced while retaining a very high selectivity to carbonylated products, when an organic phosphine belonging to a special group is used. The selectivity to a certain compound, expressed in a percentage, is defined herein as 100.times.a:b in which "a" is the amount of starting olefinically unsaturated compound that has been converted into that certain compound and "b" is the total amount of starting olefinically unsaturated compound that has been converted.